1. Field of the Invention
The present invention relates to a fusing device for an image forming apparatus. More particularly, the present invention relates to a fusing device for an image forming apparatus which has an element for inductive-heating a fusing roller and a pressure roller.
2. Description of the Related Art
Generally, an image forming apparatus such as a copier, printer, facsimile or multifunctional machine combining the functions of the previous devices in a single system, is commonly provided with a printer function.
The image forming apparatus includes a fusing device for heating a sheet of paper where a toner image is transferred, and fixing the toner image to the sheet of paper by instantly melting the toner image in a powder state. The fusing device includes a fusing roller for fusing toner on a sheet of paper and a pressure roller for pushing the sheet of paper toward the fusing roller.
The fusing roller melts toner at a predetermined temperature, for example, at 180° C. The fusing roller can be heated by means of a halogen lamp, heating coil, or an inductive-heating coil.
FIGS. 1 and 2 show the structure of an inductive-heating fusing device disclosed in U.S. Pat. No. 6,341,211, issued Jan. 22, 2002, to Tsujimoto et al., the entire contents of which are incorporated herein by reference. FIG. 1 is a vertical cross-sectional view of the inductive-heating fusing device, and FIG. 2 is a cross-sectional view of an inductive fusing portion shown in FIG. 1.
Referring to FIGS. 1 and 2, the fusing device includes a fixed hollow conductive member 12 thermally fusing toner 11 on a sheet of paper, a pressure roller 13 pushing the sheet of paper 10 containing the toner 11 toward the conductive member 12, a conveyor belt 20 carrying the sheet of paper 10 interposed between the fixed conductive member 12 and the pressure roller 13, and a coil 14 inductively heating the conductive member 12. The conveyor belt 20 moves in a direction A, and the pressure roller 13 is rotated in a direction B according to the rotation of the conveyor belt 20.
The conductive member 12 is formed of a hollow pipe, for example, a carbon steel pipe, stainless alloy pipe, aluminum pipe, or an iron pipe. The pressure roller 13 includes a cylindrical roller 15 and a silicon rubber layer 16 formed on the circumference of the cylindrical roller 15. The pressure roller 13 is pressed toward the conductive member 12 by means of a spring member (not shown).
Reference number 17 denotes a rectangular core forming a closed magnetic circuit part which penetrates the hollow portion 12a of the conductive member 12. The coil 14 is wound around the core 17. When a current passes through the coil 14, a magnetic flux is created which generates an inductive current around the conductive member 12. The core 17 can be an iron core having a high coercive coefficient used in a general transformer.
Reference number 22 denotes a heat insulating layer, which prevents heat produced from the conductive member 12 and the coil 14 from being transferred to the core 17.
The operation of the fusing device as constructed above is described in greater detail below with reference to the appended drawings.
When an alternating current having a frequency of tens or hundreds of Hz is applied from a power circuit (not shown) to the coil 14 winding around the core 17, an AC magnetic flux is generated at the core 17. The magnetic flux produces an inductive current around the conductive member 12 located adjacent to the magnetic flux. The inductive current generates heat in accordance to Joules Law as known to those skilled in the art. The conductive member 12 is heated by the low-frequency inductive heat to a temperature appropriate for fusing, which ranges, for example, between 150° C. and 200° C. When the sheet of paper 10 having the unfused toner 11 passes between the conductive member 12 and the pressure roller 13, the unfused toner 11 melts and fixes on the sheet of paper 10 by the heat of the conductive member 12 and the pressure of the pressure roller 13.
FIG. 3 illustrates another example of an inductive-heating fusing device disclosed in U.S. Pat. No. 6,341,211, referenced above. Reference numerals in FIG. 3 denote the same elements as in FIGS. 1 and 2 and further denote a nipping area 19.
While the core 17 and the conductive member 12 are rectangular in FIGS. 1 and 2, the conductive member 12′ of FIG. 3 is a cylindrical roller. Since other elements in FIG. 3 are substantially identical to those in FIGS. 1 and 2, detailed descriptions thereof will be omitted.
The fusing device applies inductive heat to the conductive member 12, however, the heat produced at a portion of the core 17 around which the coil 14 is not wound, dissipates into the air, resulting in an undesired thermal loss. The pressure roller 13, indirectly contacting the conductive member 12, also absorbs heat from the conductive member 12 while attaching the sheet of paper 10 thereto, thereby resulting in an undesired thermal loss as well.
However, in the case of a printer which requires rapid fusing heat, such as a color laser printer, high-speed laser printer or a laser printer printing large sheets of paper at a high speed, the pressure roller needs to be heated.
Accordingly, a need exists for a system and method to reduce thermal losses resulting from dissipating heat, and directing dissipating heat to efficiently heat the pressure roller where required.